3.9. desertification and erosion in basins

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Last modified 13 Apr 2011, 07:27 PM

3.9.
Desertification and erosion in basins

Desertification is a recently coined
term, whose meaning has not yet fully crystallised either from the technicians or
from societys viewpoint. According to Puigdefabrégas (1995), desertification refers
to a situation in which the lands productive capacity deteriorates and the
socio-economic systems that exploit that system start to crumble. These zones are affected
by a water shortage during at least one season of the year, are fraught with problems
resulting from negative human activity owing to over-use of the resources and finally, are
suffering from or have suffered from acutely dry periods or dry periods whose duration is
greater than normal for that region.

Desertification can cause a
reduction of infiltration into the soil and thus, a greater surface flow, with a
corresponding increase in the maximum flood discharges. Desertification also causes
modifications to the vegetation cover, which is currently undergoing rapid changes as a
result of deforestation, either for providing fuel or for obtaining more arable land. The
new vegetation cover, when it does exist, consists of either crops or poor vegetation. The
soil is unprotected and the erosion caused by an increasing surface flow on the ground
becomes even more serious, thus starting a spiralling process. Another consequence is a
reduction in the storage capacity of reservoirs which receive and retain a larger amount
of sediments. These reservoirs can produce environmental impacts downstream, given that
siltation and flow regulation do not allow the periodic flooding of plains with nutrients,
contributing to soil degradation.

Of all the countries on the Northern
shores of the Mediterranean Sea, Spain is the one whose conditions make it most
susceptible to suffer the effects of desertification, especially in the semi-arid areas,
because of the following reasons: a mountainous morphology with steep slopes, heavy
rainfall with considerable erosion ability, climatic conditions that range from sub-humid
to semi-arid, temperatures and rainfall featuring highly variations from one year to the
next, over-exploited systems because of the delicate balance between availability and
water resource consumption, etc. (Puigdefábregas, 1995).

In Portugal, the most frequent type
is water erosion because of the climatic conditions, the irregularity and concentration of
the rainy period with intensive events immediately after the dry season, when the soil is
dry and less protected. European studies, show that the most affected areas of Portugal
are the mountainous regions in the north, although the erosion phenomenon in plains such
as Alentejo or in Algarve has increased.

Considering the semi-arid areas of
Portugal, 69% of them have a high risk of erosion and only 5% a low risk.

Since the Stockholm Conference of
1972, several projects have been developed concerning erosion processes but they do not
form a global monitoring program. There are, however, some experimental centres in
Alentejo and Algarve regions (southern Portugal) where long runs of erosion and siltation
data have been measured at seven hydroelectric dams.

In Sardinia, different
desertification phenomena are caused by soil erosion. Desertification occurs after fires,
on slopes subjected to ploughing, and because of overgrazing and wood clearance. Good
farmland is also getting decreasing because of urbanisation and contamination.